1. Field of the Invention
This invention relates to the detection of gases using laser beams. More particularly, this invention relates to the remote detection of gases in the atmosphere using the outputs of excimer/dye lasers which have been frequency-shifted in circulating-media Raman cells.
2. Description of the Prior Art
The detection of gases in the atmosphere using laser beams in the infrared region is known to those skilled in the art. Canadian Pat. No. 808,760, for example, describes the detection of hydrocarbon gases using noble gas lasers such as a helium-neon laser mounted on an aircraft. The method comprises the use of two laser beams of slightly different wavelengths, either from the same laser or from different lasers. One preselected wavelength is highly absorbed by the gas to be detected while the other is not highly absorbed, thereby providing a differential coefficient. The laser beams pass through the gas in question and are reflected back to a common detection source which measures the intensities of the two beams. Any difference in the measured intensities determines the presence and quantity of the gas in question. Dust, water droplets and other light scattering materials in the atmosphere act in a similar manner on the two beams and are thus factored out.
While such detection schemes should be highly satisfactory in determining the presence or absence of preselected gases, in practice they are restricted by the limited number of wavelengths emitted by such lasers and the number of interfering gases possibly present in the atmosphere either alone or in combination with the other gases. For example, the most popular and frequently used of such lasers, the helium-neon laser, only emits 10 possible wavelengths in the frequency band where detection takes place. While these particular wavelengths have been found selective with respect to methane, they are not useful for detection of ethane, for example.
Other lasers may be substituted for the helium-neon laser to permit selective detection of other gases such as ethylene, which cannot be detected satisfactorily with the helium-neon laser. As described by E. R. Murray and J. E. van der Laan in an article entitled "Remote Measurement of Ethylene" in Applied Optics, Volume 17 at page 814 (Mar. 1, 1978) and in U.S. Pat. No. 4,450,356, for example, the detection of ethylene or other gases in the atmosphere may be accomplished by selective absorption of wavelengths emitted by a CO.sub.2 laser. Also, as taught by Alden et al. in "Remote Measurement of Atmospheric Mercury Using Differential Absorption Lidar", Optics Letters, Vol. 7, No. 5, May 1982, sulphur dioxide, nitrogen dioxide and mercury may be remotely measured in the ultraviolet spectral region using an Nd:YAG laser.
However, in such systems for the detection of gases in the atmosphere, there is still a need for more flexible laser systems which can emit a sufficient number of different wavelengths in wavelength regions for remote gas detection so that spectral matching with the desired gases may be accomplished. Some gases may interfere with the desired gases by having differential absorption coeffcients at a particular pair of wavelengths that are sufficiently large so that the gases cannot be distinguished from each other. Accordingly, more wavelengths in the desired micron wavelength region are necessary so that more particularized wavelength pairs for analysis of desired gases may be selected. One such technique is disclosed by the above-mentioned Murray et al U.S. Pat. No. 4,450,356, wherein a first CO.sub.2 laser beam is passed through a frequency doubling crystal and summed with a second CO.sub.2 laser beam. Each CO.sub.2 laser in the Murray et al system is capable of being tuned to 80 different frequencies to provide a total of 6400 frequencies for selection. Although this represents a significant improvement in frequency selection, it is desirable to provide an unlimited number of such combinations to improve measurement accuracy.
It is known that the number of wavelengths emitted by a laser source can be increased and the frequency range changed by the use of Raman shifting. For example, Alden et al disclose that Raman shifting may be used in conjunction with an Nd:YAG laser to reach the mercury absorption line. In addition, Paul Rabinowitz, Bruce Perry and N. Levinos in an article entitled "A Continuously tunable Sequential Stokes Raman Laser" describe frequency shifting of excimer/dye laser radiation using a high pressure hydrogen cell with a confocal resonator to produce multiple gain paths in a Ramanactive medium. All things being equal, the system described by Rabinowitz et al. should allow for an unlimited number of operating wavelengths, and the problem of limited target gas selection in the above-described systems would appear to be solved.
However, in the application of such technology for the detection of gases in the atmosphere, there is still a need for a system capable of making more sensitive differential measurements, for the present state of the art considers only stationary Raman-active media. In such systems with stationary Raman-active media, the excited medium may not relax before the next firing of the laser. Thus, not only is the repetition rate greatly diminished, resulting in repetition rates too low for remote measurement by aircraft, for example, but the beam may be not be effectively shifted when the Raman-active medium has not had time to relax to its ground energy level. Accordingly, lidar systems constructed using such techniques for wavelength optimization have been found to yield sensitivities lower than that useful for many applications such as remote gas measurement.
When making measurements from a rapidly moving airborne platform for gases present in the near surface atmosphere, such as those from chemical plant spills, the problems of low repetition rate lidar systems using Raman-shifting in stationary media become even more apparent. Thus, it is still necessary to increase the sensitivity and repetition rate of systems for remotely measuring gases in the atmosphere such the above-mentioned lidar systems of the prior art in order to achieve more reliable measurements.